From: Andrey Konovalov <andreyknvl@gmail.com>

When KASAN support was being added to the Linux kernel, GCC did not yet
support all of the KASAN-related compiler options. Thus, the KASAN
Makefile had to probe the compiler for supported options.

Nowadays, the Linux kernel GCC version requirement is 5.1+, and thus we
don't need the probing of the -fasan-shadow-offset parameter: it exists in
all 5.1+ GCCs.

Simplify the KASAN Makefile to drop CFLAGS_KASAN_MINIMAL.

Also add a few more comments and unify the indentation.

Signed-off-by: Andrey Konovalov <andreyknvl@gmail.com>
---
 scripts/Makefile.kasan | 43 +++++++++++++++++++++---------------------
 1 file changed, 21 insertions(+), 22 deletions(-)

diff --git a/scripts/Makefile.kasan b/scripts/Makefile.kasan
index 390658a2d5b74..04b108f311d24 100644
--- a/scripts/Makefile.kasan
+++ b/scripts/Makefile.kasan
@@ -22,30 +22,29 @@ endif
 ifdef CONFIG_KASAN_GENERIC
 
 ifdef CONFIG_KASAN_INLINE
+	# When the number of memory accesses in a function is less than this
+	# call threshold number, the compiler will use inline instrumentation.
+	# 10000 is chosen offhand as a sufficiently large number to make all
+	# kernel functions to be instrumented inline.
 	call_threshold := 10000
 else
 	call_threshold := 0
 endif
 
-CFLAGS_KASAN_MINIMAL := -fsanitize=kernel-address
-
-# -fasan-shadow-offset fails without -fsanitize
-CFLAGS_KASAN_SHADOW := $(call cc-option, -fsanitize=kernel-address \
-			-fasan-shadow-offset=$(KASAN_SHADOW_OFFSET), \
-			$(call cc-option, -fsanitize=kernel-address \
-			-mllvm -asan-mapping-offset=$(KASAN_SHADOW_OFFSET)))
-
-ifeq ($(strip $(CFLAGS_KASAN_SHADOW)),)
-	CFLAGS_KASAN := $(CFLAGS_KASAN_MINIMAL)
-else
-	# Now add all the compiler specific options that are valid standalone
-	CFLAGS_KASAN := $(CFLAGS_KASAN_SHADOW) \
-	 $(call cc-param,asan-globals=1) \
-	 $(call cc-param,asan-instrumentation-with-call-threshold=$(call_threshold)) \
-	 $(call cc-param,asan-instrument-allocas=1)
-endif
-
-CFLAGS_KASAN += $(call cc-param,asan-stack=$(stack_enable))
+# First, enable -fsanitize=kernel-address together with providing the shadow
+# mapping offset, as for GCC, -fasan-shadow-offset fails without -fsanitize
+# (GCC accepts the shadow mapping offset via -fasan-shadow-offset instead of
+# a normal --param). Instead of ifdef-checking the compiler, rely on cc-option.
+CFLAGS_KASAN := $(call cc-option, -fsanitize=kernel-address \
+		-fasan-shadow-offset=$(KASAN_SHADOW_OFFSET), \
+		$(call cc-option, -fsanitize=kernel-address \
+		-mllvm -asan-mapping-offset=$(KASAN_SHADOW_OFFSET)))
+
+# Now, add other parameters enabled in a similar way with GCC and Clang.
+CFLAGS_KASAN += $(call cc-param,asan-instrumentation-with-call-threshold=$(call_threshold)) \
+		$(call cc-param,asan-stack=$(stack_enable)) \
+		$(call cc-param,asan-instrument-allocas=1) \
+		$(call cc-param,asan-globals=1)
 
 # Instrument memcpy/memset/memmove calls by using instrumented __asan_mem*()
 # instead. With compilers that don't support this option, compiler-inserted
@@ -57,9 +56,9 @@ endif # CONFIG_KASAN_GENERIC
 ifdef CONFIG_KASAN_SW_TAGS
 
 ifdef CONFIG_KASAN_INLINE
-    instrumentation_flags := $(call cc-param,hwasan-mapping-offset=$(KASAN_SHADOW_OFFSET))
+	instrumentation_flags := $(call cc-param,hwasan-mapping-offset=$(KASAN_SHADOW_OFFSET))
 else
-    instrumentation_flags := $(call cc-param,hwasan-instrument-with-calls=1)
+	instrumentation_flags := $(call cc-param,hwasan-instrument-with-calls=1)
 endif
 
 CFLAGS_KASAN := -fsanitize=kernel-hwaddress \
@@ -70,7 +69,7 @@ CFLAGS_KASAN := -fsanitize=kernel-hwaddress \
 
 # Instrument memcpy/memset/memmove calls by using instrumented __hwasan_mem*().
 ifeq ($(call clang-min-version, 150000)$(call gcc-min-version, 130000),y)
-CFLAGS_KASAN += $(call cc-param,hwasan-kernel-mem-intrinsic-prefix=1)
+	CFLAGS_KASAN += $(call cc-param,hwasan-kernel-mem-intrinsic-prefix=1)
 endif
 
 endif # CONFIG_KASAN_SW_TAGS
-- 
2.25.1

On Wed, Aug 14, 2024 at 12:40 AM <andrey.konovalov@linux.dev> wrote:
>
> Signed-off-by: Andrey Konovalov <andreyknvl@gmail.com>

It is easier to read now, and indeed GCC 5.1+ and LLVM 13+ both
support the flags, so `CFLAGS_KASAN_SHADOW` can't be empty.

> +# First, enable -fsanitize=kernel-address together with providing the shadow
> +# mapping offset, as for GCC, -fasan-shadow-offset fails without -fsanitize
> +# (GCC accepts the shadow mapping offset via -fasan-shadow-offset instead of
> +# a normal --param). Instead of ifdef-checking the compiler, rely on cc-option.

I guess "a normal --param" means here that it is the usual way to
tweak the rest of the KASAN parameters, right?

> +# Now, add other parameters enabled in a similar way with GCC and Clang.

I think the "with" sounds strange, but I am not a native speaker.
Perhaps "in a similar way with" -> "similarly in both"?

Thanks!

Cheers,
Miguel

On Wed, Aug 14, 2024 at 5:37 PM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> On Wed, Aug 14, 2024 at 12:40 AM <andrey.konovalov@linux.dev> wrote:
> >
> > Signed-off-by: Andrey Konovalov <andreyknvl@gmail.com>
>
> It is easier to read now, and indeed GCC 5.1+ and LLVM 13+ both
> support the flags, so `CFLAGS_KASAN_SHADOW` can't be empty.
>
> > +# First, enable -fsanitize=kernel-address together with providing the shadow
> > +# mapping offset, as for GCC, -fasan-shadow-offset fails without -fsanitize
> > +# (GCC accepts the shadow mapping offset via -fasan-shadow-offset instead of
> > +# a normal --param). Instead of ifdef-checking the compiler, rely on cc-option.
>
> I guess "a normal --param" means here that it is the usual way to
> tweak the rest of the KASAN parameters, right?

Yes, clarified in v2.

> > +# Now, add other parameters enabled in a similar way with GCC and Clang.
>
> I think the "with" sounds strange, but I am not a native speaker.
> Perhaps "in a similar way with" -> "similarly in both"?

Sure, done in v2.

Thank you!